Polyurethane (PU) synthetic leathers generally comprise a fabric, a PU skin layer and a PU foam (also known as a poromeric layer) sandwiched between the fabric and the skin layer. The foam mainly contributes to softness and hand-feel of the PU synthetic leather. The skin layer can provide additional features including patterns, color, gloss, and abrasion resistance.
Currently, most PU synthetic leathers are made using volatile organic solvents such as dimethylformamide (DMF), methylethyl ketone (MEK) and toluene. These solvent-based systems are less environmentally friendly and less healthy than aqueous systems. Therefore, aqueous systems are more desirable.
Attempts have been made to minimize the use of volatile organic solvents in the manufacturing of PU synthetic leather. WO2004061198A1 discloses a method for preparing a synthetic leather including impregnating or coating a non-woven or woven textile with an aqueous polyurethane dispersion (also known as PUD) to form a poromeric layer. The prior art does not disclose how to prepare synthetic leathers comprising an outer layer that is also made from aqueous compositions.
In preparing solvent-based PU synthetic leathers, a release paper process is one widely used approach. The release paper process typically comprises the steps of: (1) providing a PU foam coated on a fabric, which is made using an organic solvent; (2) applying a solvent-based PU resin paste to a release paper to form a skin layer; (3) attaching the foam to the release paper with the skin layer therebetween to form a multilayer sheet; (4) peeling off the release paper from the multilayer sheet to obtain a synthetic leather with a surface that has a profile corresponding to that of the release paper. When an embossed release paper is used, the resultant PU synthetic leathers have an embossed surface, which is desirable for many applications for advantageous visual appearance.
Compared to solvent-based compositions used to form a skin layer, aqueous compositions usually have poorer coatability on release paper because release paper is hydrophobic. As a result, synthetic leathers made thereform may have a flawed surface, for example, holes or cracks on the surface. Thus, it is challenging to use the release paper process to prepare synthetic leathers which have both the foam and the skin layer made from aqueous compositions.
Moreover, replacing solvent-based compositions with aqueous compositions may have undesirable effects on mechanical properties of synthetic leathers. Synthetic leathers made from aqueous compositions contain surfactants and can be free of organic solvents. Both migration of surfactants to the interface of different layers, and an absence of organic solvents that can cause the molecules of polymer layers to interpenetrate may cause the synthetic leather to have lower interlayer adhesion strength than solvent-based PU synthetic leathers. In addition, aqueous compositions generally form films with lower film strength than that of solvent-based compositions, which may compromise bally flex property and/or abrasion resistance of the resultant synthetic leathers.
The PU synthetic leather industry requires synthetic leathers having certain adhesion strength and bally flex property to meet national and/or industry standards. The adhesion strength, that is, interlayer adhesion strength between any two adjacent layers of the synthetic leather, should be 1,000 gram per centimeter of sample width (g/cm) or more as measured by Method SLF11 adopted by the Society of Leather Technologists and Chemists. To withstand frequent bending during applications, PU synthetic leathers also need to pass 10,000 times of bally flex test as measured by China's GB/T 8949-1995. It is also desirable that PU synthetic leathers have sufficient abrasion resistance for use in some applications such as in automotives.
Therefore, it is desirable to provide a process for preparing a multilayer structure suitable for use as a synthetic leather, which is carried out free from organic solvents. It is desirable that such multilayer structure comprises a PU foam and a skin layer both made from aqueous compositions, and can be free from surface flaws. At the same time, it is desirable for such multilayer structure to have the previously described adhesion strength and bally flex property to meet national and/or industry standards. Moreover, it is desirable to provide a multilayer structure having an embossed surface while still having the previously described adhesion strength and bally flex property.